Normal human erythrocytes bind approximately 210,000 molecules of Robinia phytohemagglutinin with an apparent association constant of 2.7 × 106 m-1. Neuraminidase treatment of the cells increased Robinia PHA binding by 50-60% while trypsin treatment of the cells decreased the amount of PHA bound by about 35%. A highly purified glycopeptide derived from the trypsin released glycopeptides was shown to have potent Robinia PHA haptene inhibitory activity. The specificity for binding to Robinia PHA residues in the oligosaccharide portion of the glycopeptide with the determinant sugars being the galactose residues of the outer branches and the mannose residues of the core. Competitive binding studies indicate that the Robinia PHA binding sites on human erythrocytes also serve as Ph. vulgaris PHA receptor sites while at least half of the Ph. vulgaris PHA binding sites are inaccessible to Robinia PHA.